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We perform simulations of giant impacts onto the young Uranus using smoothedparticle hydrodynamics (SPH) with over 100 million particles. This100--1000$\times$ improvement in particle number reveals that simulations withbelow 10^7 particles fail to converge on even bulk properties like thepost-impact rotation period, or on the detailed erosion of the atmosphere.Higher resolutions appear to determine these large-scale results reliably, buteven 10^8 particles may not be sufficient to study the detailed composition ofthe debris -- finding that almost an order of magnitude more rock is ejectedbeyond the Roche radius than with 10^5 particles. We present two softwaredevelopments that enable this increase in the feasible number of particles.First, we present an algorithm to place any number of particles in a sphericalshell such that they all have an SPH density within 1% of the desired value.Particles in model planets built from these nested shells have aroot-mean-squared velocity below 1% of the escape speed, which avoids the needfor long precursor simulations to produce relaxed initial conditions. Second,we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT usestask-based parallelism and other modern algorithmic approaches to take fulladvantage of contemporary supercomputer architectures. Both the particleplacement code and SWIFT are publicly released.

Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT